In the art, and in particular from FR-A-2,654,978, it is already known to be possible to use, in this field, resistances in the form of a meshed network making up an electrically-conducting net extending, with electrical continuity, from one end of this net to an opposite end.
In order to carry out the welding, the net is placed in the location of the welding zone of the elements, and is connected via connection terminals to an electrical power source intended to cause to flow, in the resistance, a current of sufficient strength to raise the temperature of the surrounding part of these elements up to a predetermined temperature corresponding to their melting point, thus ensuring their cohesion under pressure and then their welding, after stopping of the current supply and cooling.
In the abovementioned publication FR-A-2,654,978, the resistive net is most particularly intended for the electric welding of thermoplastic tubes which are used in the gas industry and are joined by a connection element made from compatible material, whether a sleeve for joining two coaxial main pipes or a junction saddle (also called branch connector) for transverse taps or branch-offs, for example those in the form of a "T".
The welded connection of two coaxial main pipes by means of a heating-net sleeve poses virtually no problem in principle, it being possible for the said net in particular to be presented as a length of a cylinder embedded close to the inner surface wall of the sleeve, with its cylinder axis substantially coincident with that of the sleeve (which is normally also substantially cylindrical).
However, it has proved to be the case that further improvements can be made when the heating net equips a branch connector for the transverse connection of a main pipe to another pipe.
In order for the problem posed to be well understood, let us briefly recall the possible make-up of such a "connector", two embodiments of which may be found: in the abovementioned publication and, for example, in U.S. Pat. No. 4,684,417.
Normally, these connectors, generally made of thermoplastic material, comprise a saddle-shaped part or body straddling quite closely the main pipe from which the transverse tap is to be effected.
On the convex outer surface side of the saddle there stands up a shaft which is penetrated, as is the body of the connector, by a passageway which itself can be connected to a transverse branch channel.
Once the branch connector is welded on above the length of pipe, the wall of the latter is pierced, by means of a sealer-perforator moving in the passageway of the shaft, after which the perforator is withdrawn or raised in the said passageway, beyond its communication with the branch channel on which the connection pipe may have been preinstalled, which pipe may thus communicate with the main pipe via the transverse piercing which has been made.
In fact, a problem may arise during the piercing or after this in the case where a heating net has been used to heat fuse the connector and the main pipe, as will be understood hereinbelow with reference to the attached FIGS. 1 and 2 which illustrate the prior art.
First of all, in FIG. 1, a branch connector 1 may thus be seen with its connection saddle 3 straddling a main pipe 5 in which provision may be made, for example, for a pressurized fluid, such as gas, to flow.
On the outside, above the saddle, there stands up the shaft 6 of the connector with its vertical inner passageway 7 at an intermediate level from which emerges the transverse branch channel 9 which communicates here with the inner volume of the main pipe via the piercing 11 which has been made in the wall of the latter by the sealer-perforator (not shown).
In order to produce the transverse piercing 11, the perforator has to pierce and penetrate the welding resistance 13, here embedded in the body of the saddle 3, in the immediate proximity of its concave inner surface which is substantially in the form of a portion of a cylinder, which portion is designated by 3a.
FIG. 2 illustrates, in solid lines, the substantially semi-cylindrical shape of the resistance 13, before action by the perforator.
It should thus be noted that this resistance is in the form of a meshed net having an electrically-conducting wire or wires 15 extending continuously between its two opposite ends 15a, 15b. For the welding of the saddle and the main pipe, these ends 15a, 15b are normally each connected to a connection terminal (such as 17, 19), which terminals are themselves connected to a power source, such as a direct-current generator, shown diagrammatically at 21 (and equipped, of course, with all the devices necessary for suitably supplying this resistance for the chosen period).
Thus, at the moment of welding, and until the perforator has acted, the resistive net 13 has no orifice penetrating it, this having the advantage that, at the moment of welding, it still has its electrical continuity between its wire ends 15a, 15b.
On the other hand, once the perforator has produced the piercing 11, the net then necessarily has a hole made along the axis of the perforator and of the piercing, such a hole having been shown diagrammatically at 23 and delimited by the dot-dash lines in FIG. 2.
The net thus no longer has its electrical continuity.
Moreover, it has been necessary to exert an additional force on the perforator in order to penetrate the net, with the risk of pulling certain sections of wire (wires), causing potential passageways for leaks.
Furthermore, it turned out that the operations for checking the tightness of the joints between the connectors and the main pipes were made more tricky because of the presence of a net portion along the axis of the passageway 7 via which the operator conducts some of these operations before piercing of the hole 11.
Still within the scope of the transverse connection, in particular of tubes, it has also already been proposed to use a plastic ring or collar integrating a resistance which is most often spiralled, but which may occasionally assume a corrugated configuration (see especially the publication JP-A-3,071,824 and JP-A-2,256,997).
However, even if by virtue of such a crown-shaped plastic element it is possible for the resistance to preserve electrical continuity, this solution necessitates manufacturing an additional element (the crown) with the corresponding steps for installing and monitoring the resistance which has to be embedded therein. Moreover, as a matter of fact, the additional cost will be even greater since it will be necessary to provide virtually as many crowns of different dimensions as there may be diameters of connection orifices to be pierced, or indeed of tube diameters.